1 Technical Field
The present invention relates to stators for electric rotating machines which include a stator core having a yoke portion and a plurality of tooth portions that are separately formed from and assembled to the yoke portion.
2 Description of Related Art
In recent years, AC (Alternating Current) motors, such as induction motors and synchronous motors, have been required to be small in size and high in output.
To meet such a requirement, there is disclosed, for example in Japanese Patent Application Publication No. H07-111746, a method of manufacturing a stator for an AC motor.
Specifically, referring to FIG. 11, according to the method, a stator core 100 is configured to include an annular yoke portion 110 and a plurality of tooth portions 120 that are separately formed from the yoke portion 110. The yoke portion 110 has a plurality of recesses 111 formed in a radially inner surface thereof. On the other hand, each of the tooth portions 120 has a protrusion 121 formed at a radially outer end thereof. In manufacturing the stator, a stator coil (not shown) is first wound around the tooth portions 120 at high density. Then, the tooth portions 120 are assembled to the yoke portion 110 by fitting the protrusions 121 of the tooth portions 120 respectively into the recesses 111 of the yoke portion 110 in the axial direction of the yoke portion 110. Consequently, the stator is obtained which includes the stator core 100 and the stator coil mounted on the stator core 100.
Moreover, according to the method, for facilitating the process of assembling the tooth portions 120 to the yoke portion 110, the stator core 100 is configured to further include an annular connecting portion 130 that circumferentially extends to connect together all the radially inner ends of the tooth portions 120.
However, with the above configuration of the stator core 100, during operation of the motor, part of magnetic flux may flow through the connecting portion 130 in the circumferential direction of the stator core 100; the magnetic flux is created, by current flowing through the stator coil, on the radially outer periphery of a rotor (not shown) of the motor which is disposed radially inside the stator. Consequently, the connecting portion 130 may be magnetically saturated, thus undesirably affecting the distribution of magnetic flux in the motor and thereby resulting in torque ripple of the motor. As a result, when the motor is employed in an electric steering system of a motor vehicle so that the torque generated by the motor is directly transmitted to the steering wheel of the vehicle, the torque ripple will deteriorate the steering feeling of the driver.